Sammanfattning: District heating can contribute to increased energy efficiency in society. The environmental impact can be reduced by minimising the heat losses in the district heating pipes during distribution of the heat to the customers. Today, the most commonly used type of district heating pipe has a steel or copper media pipe, surrounded by polyurethane (PUR) foam insulation and a protective outer casing of polyethylene. The heat losses increase over time due to diffusion of blowing agents out of and air into the insulating foam. In this thesis, the long-term environmental and thermal performance of different insulating materials was studied. The diffusion mechanisms of cyclopentane, a commonly used blowing agent in PUR foam, were compared to those of a new gas, 1,1,1,3,3-pentafluorobutane (HFC-365mfc). The performance of polyethylene terephthalate (PET) foam as a possible replacement alternative to PUR foam was also investigated. The environmental impact from global warming, acidification and resource depletion was studied for the first three phases (pipe production, network construction and network use) of the life-cycle of different pipe alternatives by means of life-cycle assessment (LCA). The LCA-study of PET foam insulated pipes is theoretical, since only foam boards can be produced today. The diffusion characteristics of HFC-365mfc in PUR foam were found to be similar to those of cyclopentane. District heating pipes insulated with HFC-365mfc blown PUR foam may have less environmental impacts due to heat losses than cyclopentane blown foam, but the high global warming potential of HFC-365mfc render its use questionable. If it is assumed that the entire HFC-365mfc content in the PUR foam were to be released to the atmosphere, the use of this gas for pipe insulation can not be justified as an option to cyclopentane blown foam. The determined effective diffusion and solubility coefficients of blowing agents and air are lower in PET foam than in PUR foam. The environmental performance of pipes insulated with high-density carbon dioxide blown PET foam (157 kg?m3) is similar to pipes insulated with carbon dioxide blown PUR foam (86 kg?m3). If PET foam of lower density could be produced, it would be a potential competitor to pipes insulated with cyclopentane blown PUR foam. A future trend towards increased recycling of PET can be expected in Europe, possibly as a result of increased PET consumption and current regulations. Recycled PET has also the advantage of a lower price than virgin PET and PUR.